I have a pretty fair understanding of how transmissions work but one thing has always puzzled me. Suppose you are driving along the road and the light turns red. You stop. Obviously the wheels aren’t moving but your engine still is. What is going on with the transmission at this point when you have stepped on the brakes and your car is still on? The output shaft of the transmission is not moving because your wheels aren’t moving but the input shaft from the flywheel is still moving since your engine is still running. Correct me if I am mistaken. Can someone explain to me where the rotation of the input shaft goes to if your car is stationary (aka the output shaft isn’t being moved)? How does this differ in a manual versus an automatic transmission?
DUP please see other copy of this question.
I couldn’t find a duplication of this post so I’ll answer it.
An automatic transmission has a torque converter. This is a fluid coupling. This allows the outer part of the torque converter which is attached to the engine to spin while the inner part of the torque converter doesn’t when the vehicle is stopped. You can demonstrate this by using two fans. Point the two fans at each other and only turn on one. The fan that is running will push air past the one that isn’t. That fan begins to spin from the air from the running fan until it’s spinning at the same rate. Now take a pencil poke it into the fan that isn’t running so the blades stop spinning. That’s basically how a torque converter in an automatic transmission works.
Tester
Good explanation from Tester. Now I know what you’re thinking. What makes this FORCE movement.
Well, a propeller gets a boat to move, doesn’t it? Why doesn’t it just sit there ejecting water out the back and sit still?
Btw-there has to be a differential for any power transmission across a fluid coupling. The lock up converter eliminates this fundamental characteristic.
Just a nitpick, Tester…
Thanks to the evolution of dual clutch automatics, not all automatic transmissions have torque converters anymore. Instead, they’re really just like a manual transmission, and the clutch simply disengages at a stop light.
The following vehicles available in the US use this type of automatic on at least some of their automatics (I believe this list is correct):
Audi A3
Audi S4
Audi A5/S5
Audi TT
BMW M3
BMW Z4
Ferrari 458 Italia
Ford Fiesta
Mercedes SLS AMG
Mitsubishi Lancer Evolution MR
Mitsubishi Lancer Ralliart
Nissan GT-R
Porsche 911
Porsche Cayman
Porsche Boxster
Porsche Panamera
Volkswagen Passat
Volkswagen Jetta
Volkswagen Eos/Golf
Volkswagen CC
I know the 2012 Ford Focus is supposed to get one of these as well, but I can’t remember what else is coming out with one soon…
“there has to be a pressure differential…”
Just to keep it from being confused with the car’s differential.
Oh I see, but I’m still a little confused. What is happening to the moving fluid in the torque converter due to the engine that is still running? Does it simply not affect the part of the torque converter that is attached to the transmission shaft? Won’t the moving fluid in the torque converter due to the running engine cause motion inside the torque converter which would cause pressure buildup? Please correct me if I am wrong.
Paperclip, please do a search on google, there are many excellent web sites with animations, etc. An automatic transmission is better explained with pictures, anyway.
In a manual transmission when you are at a stop and with clutch disengaged (pedal down), the input shaft of the transmission is stationary if the transmission is still in gear. If the transmission is in ‘neutral’ and the clutch disengaged, the input shaft can rotate if there is enough drag caused by the pilot bearing and clutch disk. With the transmission in ‘neutral’ and the clutch engaged (pedal up), the input shaft of the transmission is turning but no gear is engaged so no power is transferred.
In an automatic when the transmission is in gear, engine running, and vehicle stopped, the input shaft is not turning. The pump inside the shell of the torque converter is slinging the oil contained inside outward and with the rotation of the engine. The turbine’s vanes are shaped to catch the oil turn it inward and counter to the engine rotation producing a momentum change that results in a force. This force develops the torque that is transfered to the input shaft but the torque cannot do any work because the brakes are ‘on’. As the oil exits from the turbine in the center of the TC, the flow angles would be wrong to the passing pump vanes and much turbulence would be created – think heat generated. Instead, there is a third element, the stator that redirects the oil flow so that the counter rotation is sent pro rotation smoothly into the pump vanes. When you hold the brakes and run the engine up against the transmission, a torque multification of approximately 2.3 is attained. However, since no work is being done on the car all the engine power is going into the torque converter fluid causing friction heat (that oil is really moving). Thus, the fluid out of the torque converter is sent to the radiator heat exchanger to cool it down before returning to the sump pan for reuse. BTW, the hub of the torque converter which is part of the shell driven by the engine flex plate (flywheel) drives the line pressure oil pump that hold the clutches inside the transmission together engaging first or reverse.
Have any other questions, feel free to ask.
“However, since no work is being done on the car all the engine power is going into the torque converter fluid causing friction heat (that oil is really moving). Thus, the fluid out of the torque converter is sent to the radiator heat exchanger to cool it down before returning to the sump pan for reuse.”
THANK YOU. That is the answer to my question. So to get this straight, the motion inside the torque converter due to the engine power when the car is at rest is sent to the radiator and the heat generated is dissipated in the radiator is that correct? If this is the case, then this means that the torque converter is not a completely sealed system because there is fluid moving to and coming in from the radiator. Is this also correct? I guess the reason why I am having trouble visualizing this is because I always imagined a torque converter as a fully sealed, fully pressurized system with no fluid moving elsewhere except for within the torque converter. Please let me know if I am still not understanding this correctly. THANKS SO MUCH.
The amount of oil being pumped into and exiting out of the torque converter is miniscule compared to the amount of oil circulating inside. So for practical purposes the TC is sealed. The line pressure pump builds line pressure. Excess oil bled from the line pressure regulator goes to the torque converter pressure regulator which sets the oil pressure inside the torque converter. Oil flows into the TC through the center of the input shaft and exits through the stator support shaft to the oil cooler out line. The return line from the radiator goes to the sump (pan). When you do a stall test of an automatic i.e. shift to ‘drive’ or ‘reverse’; lock the brakes; run the engine to full throttle; read the RPM; and then lift the throttle, you have to idle the engine for a few minutes to cool the TC before doing a test in another gear.
If a car has sat for a long spell (couple of days), the TC will partially drain back to the sump. It will take a minutes or so in ‘neutral’ to refill the TC. During that time span the TC will not transfer torque efficiently i.e. you start the car; shift immediately to drive; and get a neutral condition until the TC refills.
Think of a torque converter as a hollowed out bagel with vanes inside and filed with fluid. When the front half spins, the fluid via the vanes draws the back half along, unless the back half is held from rotating by something like brakes. If so, the energy is converted in the fluid into heat energy and dissipated by the tranny cooling system.
It’s like Geesa’s boat comments. If the propeller is turning slowly and the boat is tied to the dock, the energy generated by the propeller that would normally move the boat forwrad is simply dissipated by the water. The boat goes nowhere. The engine half of the torque converter is like the propeller, the brakes are like the rope to the dock, and the fluid in the torque converter is like the water in the lake.
Eraser’s comment is a good one, but it does not apply to your Camry. You have a good old fashioned torque converter.
Thanks! That really helped. So is the fluid inside the torque converter the same thing as what is known as the “transmission fluid” or is it a different fluid? Can you explain the flow and usage of the transmission fluid throughout the car? I believe I read somewhere that the transmission fluid is used the lubricate the gears in the actual tranny as well. Also, when we are checking the transmission fluid using the transmission fluid dipstick, what part of the transmission is the dipstick being pulled out of/stuck into?
“The return line from the radiator goes to the sump (pan)”
This is not the same thing as the oil pan for the engine is it? Where is the sump for the transmission located at?
Yup, it’s the same fluid.
It’s used not only to transfer energy in the manner described, but also to lubricate, carry away heat, and as a hydraulic fluid to opeate the mechanical controls inside the tranny that make it work. It has multitudinous responsibilities.
No the tranmission pan (sump) is separate from the engine pan (sump). It would be engine oil in the engine pan and transmission fluid (oil) in the transmission pan. The transmission dipstick ends up measuring the level of the fluid in the transmission pan. The line pressure oil pump sucks up the oil from the transmission pan through a filter/screen and sends that oil to the valve body to pressurize clutches and bands, fill and exchange the torque converter fluid, lubricate the gears, bearings, clutch frictions and plates and brake bands, remove heat from the Torque converter and friction elements.
BTW, the reason the sump level should be at the correct level is that if it is too low the line pressure pump would draw air and possibly completely lose pressure. That makes for slippage, overheated clutch frictions and plates, and neutraling. Too full, and the rotating components, clutch shells, ring gears, etc. will be spinning in and churning the oil and aerating it. That makes for slipping, poor shift quality, and regulator buzzing.
Also, when we are checking the transmission fluid using the transmission fluid dipstick, what part of the transmission is the dipstick being pulled out of/stuck into? Thanks
The case, not the torque converter, but I’ll ask someone here to add specifics. I’m not really a tranny guy. Someone who’s torn some various iterations apart can probably be more specific.
Thanks. You have been really helpful. Do you have any suggestions as to how I can learn more about cars? How did you end up knowing so much? I don’t really have a car I can take apart and look at so pretty much everything I learn is through the Internet
Paperclip, I just gotta ask - why get this information piecemeal over days when you could get in 30 minutes with a few Google searches?